The Journal of Indian Botany/Volume 3/October 1922/Mixed Formations in Time : A New Concept in Oecology

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MIXED FORMATIONS IN TIME : A NEW CONCEPT

IN OECOLOGY

BY

W. T. Saxton, M.A., F.L.S.

(With one diagram)

1. Introduction.

The writer has always deprecated the action of those persons who evolve new theories* from a priori considerations, out of their own inner consciousness, without any material evidence to support them, as well as of those who glean some facts from half a dozen published sources, diligently select those favourable to a particular pet theory, ignore all the other facts which do not fit it and rush into print.

It seems, therefore, advisable to state that the theory to be developed here has not arisen in either of these two ways. It is based mainly on some oecological work which is being carried on at the present time by the writer and some of his students. This work is likely to be continued for some months at least and meanwhile the idea gradually crystallizing from it seemed sufficiently important, in its relation to Indian oecological problems, to justify this attempt to complete the crystallizing process. It is proposed rather to explain the idea than to give details of the facts which first led to its adoption.

2. Discussion.

Probably many Indian Botanists have been struck by the extreme paucity of references to India in oecological literature. Warming’s classical "Oecology of Plants" contains practically no reference to India except a few passing remarks about “Monsoon Forest” (which he quite possibly classifies incorrectly). I know of no work in recent years which has suggested any broad basis upon which detailed oecological studies in India may rest. Consequently such oecological work as has been done, apart from any question of its intrinsic merit, has no basis of correlation except European and American work. The result has been that those who have tried to define Indian plant communities have quite definitely made the attempt to fit them into the systems which have evolved in the last twenty years from the oecological work done in Western countries. Those systems are already comparatively rigid and the task of fitting our plant communities into them is no* an easy one. Indeed I have come to the conclusion that it cannot be done, at least not on the lines which have been hitherto attempted. From the oecologieal point of view the great difference between Europe and America on the one hand and India on the other is that the former really have no habitat which even approximately corres- ponds to the Indian monsoon habitat, with its succeeding eight months of drought. This is the one great factor which has been, in certain aspects, unconsciously ignored, and which cannot be ignored. Seasonal differences are found in other habitats, but I believe it must be admitted that they are much less profound and far reaching than those in the monsoon habitat as we see it in its typical development.

When we examine such a habitat we often find that during the year two (or it may be three) profoundly different sets of conditions are met with in the same spot. Practically these constitute two (or three) entirely different habitats . First a condition when both soil and air may be almost continuously water saturated for about 3 months, and when comparatively delicate mesophytic plants may flourish. If towards the end of this period the soil becomes com- pletely water-logged, aeration soon becomes very deficient, which upsets the equilibrium in such a way that swamp conditions arise. Whether this happens or not, and how soon, will depend, amongst other things, on the sub-soil drainage. In any case at the close of the monsoon the soil and air both gradually dry, and after about a month a period of seven months intense and absolute drought sets in. During this time the formation to be met with must be regarded as most markedly xerophytic. Only plants with some well defined x9rophytic equipment can survive.

Now according to our present conceptions we think of any parti- cular small area, if reasonably stable, as being occupied by one forma- tion, though doubtless it may have different seasonal aspects and different floristic composition at different times. Still, essentially, we think of it as a unit*

But in such habitats as have been described above such a concep- tion often appears absolutely untenable and absurd. Examining a hypothetical small area, such as I have mentioned, it may be supposed to be xerophytic bushland in the months of April and May, equally well to be a meadow* with scattered bushes in July and August, and possibly also to be a marsh in September and October.

The idea of 14 succession ” is of course familiar, but this is not

• I use the term meadow here and elsewhere merely in an illustrative

manner, without wishing to imply : that there is anything in the plains of India which exactly corresponds to the ‘ meadow ’ of North Temperate coun- tries, which is in fact very doubtful. succession as understood in Oecology. Probably this will be universally admitted and it will not be necessary to discuss the point further.

Prom what has been said it follows that the small area in ques- tion may not be at any time (and probably in fact is not) a pure formation, i.e., pure meadow, or pure marsh, or pure xerophytic bushland. The latter will be most nearly pure of the three, say in May, but the ground will still contain the resting organs (rhizomes, etc.) of the plants of the other two formations and possibly a very few hardy and persistent survivals of the marsh. So that in fact our area bears a mixture of three (or two if marsh does not develop) formations, the relative importance of which varies with the progression of time. This is the idea which forms the title of this essay “ Mixed formations in time It is necessary to distinguish the case quite clearly from the already familiar one of a mixed formation between two pure ones, in a relatively narrow zone where they overlap. Eor instance wood- land instead of passing over abruptly into pure grassland may first pass into an intermediate mixed zone of grassland with scattered trees. This is quite distinctly a case of mixed formations in space . Time does not effect the relationship in any regularly recurrent manner.

The diagram appended is meant to be no more than hypothetical, but it may serve to visualize the idea of mixed formations in time. The plain dotted line represents the relative dominance of the xerophytic formation, the — line shows the same for the mesophytic and the — •• — •• — line the relative dominance of helophytic vegetation. One point in the diagram may require emphasis, the vertical ordinate at any point is not meant to represent absolute dimensions, but only relative dominance.

DIAGRAM TO ILLUSTRATE MIXED FORMATIONS IN TIME.

Apart from the general argument I have followed, there appear to be definite facts, derived from a study of the physiological anatomy of plants of such a mixed formation, which lend support to the theory. It is not my purpose to state or discuss them at present, especially as they are very incompletely worked out. If I have said enough to indicate clearly what the new theory is which I am bringing forward, and in what way it promises to be of service to Indian oecology, then my task is completed for the present. I trust the statement may lead to criticism and discussion.

Summary.

Certain familiar facts are briefly described regarding climatic conditions in those parts of the world where a strong summer monsoon rain is experienced, alternating with a long and severe drought.

The theory is advanced that in such an area it often happens that we cannot regard a small and reasonably homogeneous part of it as occupied by a single unit (“ formation ”) of vegetation, but rather that two (or even three) entirely different plant communities regularly alternate with one another, though each persists to some extent through the dominant phases of the other, thus giving rise to the idea of “ Mixed formations in time.”

It is suggested that Indian oncological conditions differ so widely from those of Europe and America that it is almost useless to try and fit our oecological results into their systems. India will have to evolve her own system of oecological classification.

Madhavlal Ranchhodlal Science Institute,

Ahmed abad,

11th August , 1922.